In order to measure the PVT properties of a hydrocarbon reservoir fluid, a sample of the reservoir fluid is taken and analysed under reservoir pressure and temperature. A brief description of the way in which a PVT analysis is carried out is given in section 3 of the book Contributions in Petroleum Geology and Engineering, Volume 5, Properties of Oils and Natural Gases, K. S. Pederson et al, 1989. Such an analysis can be very accurate, however it takes a long time to be completed.
It is of great importance to know the PVT properties of the reservoir fluid as soon as possible, preferably directly after a well has been drilled. Knowing such information allows for the adjustment of the design of the production and surface equipment to take into account the actual PVT properties.